Magnetic anisotropy of soft-underlayer induced by magnetron field

ABSTRACT

A perpendicular magnetic recording medium having a substrate and a magnetic underlayer on the substrate, the magnetic underlayer having an easy axis of magnetization substantially directed in a radial or transverse direction, and a process for manufacturing the perpendicular magnetic recording medium are disclosed.

RELATED APPLICATIONS

This application claims priority from Provisional Application Ser. No.60/221,458 filed Jul. 25, 2000, entitled “Uniaxial Anisotropy ofSoft-Underlayer Induced by Magnetron Field,” the entire disclosure ofwhich is hereby incorporated herein by reference. This application isalso related to the application entitled “Magnetic Anisotropy ofSoft-Underlayer Induced by Seedlayer,” filed along with thisapplication, the entire disclosure of which is hereby incorporatedherein by reference.

FIELD OF INVENTION

This invention relates to perpendicular recording media, such as thinfilm magnetic recording disks having perpendicular recording, and to amethod of manufacturing the media. The invention has particularapplicability to high areal density magnetic recording media exhibitinglow noise.

BACKGROUND

The increasing demands for higher areal recording density imposeincreasingly greater demands on thin film magnetic recording media interms of remanent coercivity (Hr), magnetic remanance (Mr), coercivitysquareness (S*), medium noise, i.e., signal-to-medium noise ratio(SMNR), and narrow track recording performance. It is extremelydifficult to produce a magnetic recording medium satisfying suchdemanding requirements.

The linear recording density can be increased by increasing the Hr ofthe magnetic recording medium, and by decreasing the medium noise, as bymaintaining very fine magnetically non-coupled grains. Medium noise inthin films is a dominant factor restricting increased recording densityof high-density magnetic hard disk drives, and is attributed primarilyto inhomogeneous grain size and intergranular exchange coupling.Accordingly, in order to increase linear density, medium noise must beminimized by suitable microstructure control.

According to the domain theory, a magnetic material is composed of anumber of submicroscopic regions called domains. Each domain containsparallel atomic moments and is always magnetized to saturation, but thedirections of magnetization of different domains are not necessarilyparallel. In the absence of an applied magnetic field, adjacent domainsmay be oriented randomly in any number of several directions, called thedirections of easy magnetization, which depend on the geometry of thecrystal. The resultant effect of all these various directions ofmagnetization may be zero, as is the case with an unmagnetized specimen.When a magnetic filed is applied, the domains most nearly parallel tothe direction of the applied field grow in size at the expense of theothers. This is called boundary displacement of the domains or thedomain growth. A further increase in magnetic field causes more domainsto rotate and align parallel to the applied field. When the materialreaches the point of saturation magnetization, no further domain growthwould take place on increasing the strength of the magnetic field.

The ease of magnetization or demagnetization of a magnetic materialdepends on the crystal structure, grain orientation, the state ofstrain, and the direction and strength of the magnetic field. Themagnetization is most easily obtained along the easy axis ofmagnetization but most difficult along the hard axis of magnetization. Amagnetic material is said to posses a magnetic anisotropy when easy andhard axes exist. On the other hand, a magnetic material is said to beisotropic when there are no easy or hard axes.

“Anisotropy energy” is the difference in energy of magnetization forthese two extreme directions, namely, the easy axis of magnetization andthe hard axis of magnetization. For example, a single crystal of iron,which is made up of a cubic array of iron atoms, tends to magnetize inthe directions of the cube edges along which lie the easy axes ofmagnetization. A single crystal of iron requires about 1.4×10⁵ ergs/cm³(at room temperature) to move magnetization into the hard axis ofmagnetization, which is along a cubic body diagonal.

The anisotropy energy U_(A) could be expressed in an ascending powerseries of the direction cosines between the magnetization and thecrystal axes. For cubic crystals, the lowest-order terms take the formof Equation (1),U _(A) =K ₁(α₁ ²α₂ ²+α₂ ²α₃ ²+α₃ ²α₁ ²)+K ₂(α₁ ²α₂ ²α₃ ²)  (1)

where α₁, α₂ and α₃ are direction cosines with respect to the cube, andK₁, and K₂ are temperature-dependent parameters characteristic of thematerial, called anisotropy constants.

Anisotropy constants can be determined from (1) analysis ofmagnetization curves, (2) the torque on single crystals in a largeapplied field, and (3) single crystal magnetic resonance.

The total energy of a magnetic substance depends upon the state ofstrain in the magnetic material and the direction of magnetizationthrough three contributions. The first two consist of the crystallineanisotropy energy of the unstrained lattice plus a correction that takesinto account the dependence of the anisotropy energy on the state ofstrain. The third contribution is that of the elastic energy, which isindependent of magnetization direction and is a minimum in theunstrained state. The state of strain of the crystal will be that whichmakes the sum of the three contributions of the energy a minimum. Theresult is that, when magnetized, the lattice is always distorted fromthe unstrained state, unless there is no anisotropy.

“Magnetostriction” refers to the changes in dimension of a magneticmaterial when it is placed in magnetic field. It is caused by therotation of domains of a magnetic material under the action of magneticfield. The rotation of domains gives rise to internal strains in thematerial, causing its contraction or expansion.

The requirements for high areal density impose increasingly greaterrequirements on magnetic recording media in terms of coercivity,remanent squareness, low medium noise and narrow track recordingperformance. It is extremely difficult to produce a magnetic recordingmedium satisfying such demanding requirements, particularly ahigh-density magnetic rigid disk medium for longitudinal andperpendicular recording. The magnetic anisotropy of longitudinal andperpendicular recording media makes the easily magnetized direction ofthe media located in the film plane and perpendicular to the film plane,respectively. The remanent magnetic moment of the magnetic media aftermagnetic recording or writing of longitudinal and perpendicular media islocated in the film plane and perpendicular to the film plane,respectively.

A substrate material conventionally employed in producing magneticrecording rigid disks comprises an aluminum-magnesium (Al—Mg) alloy.Such Al—Mg alloys are typically electrolessly plated with a layer of NiPat a thickness of about 15 microns to increase the hardness of thesubstrates, thereby providing a suitable surface for polishing toprovide the requisite surface roughness or texture.

Other substrate materials have been employed, such as glass, e.g., anamorphous glass, glass-ceramic material which comprises a mixture ofamorphous and crystalline materials, and ceramic materials.Glass-ceramic materials do not normally exhibit a crystalline surface.Glasses and glass-ceramics generally exhibit high resistance to shocks.

FIG. 1 shows the schematic arrangement of a magnetic disk drive 10 usinga rotary actuator. A disk or medium 11 is mounted on a spindle 12 androtated at a predetermined speed. The rotary actuator comprises an arm15 to which is coupled a suspension 14. A magnetic head 13 is mounted atthe distal end of the suspension 14. The magnetic head 13 is broughtinto contact with the recording/reproduction surface of the disk 11 Therotary actuator could have several suspensions and multiple magneticheads to allow for simultaneous recording and reproduction on and fromboth surfaces of each medium.

An electromagnetic converting portion (not shown) forrecording/reproducing information is mounted on the magnetic head 13.The arm 15 has a bobbin portion for holding a driving coil (not shown).A voice coil motor 19 as a kind of linear motor is provided to the otherend of the arm 15. The voice motor 19 has the driving coil wound on thebobbin portion of the arm 15 and a magnetic circuit (not shown). Themagnetic circuit comprises a permanent magnet and a counter yoke. Themagnetic circuit opposes the driving coil to sandwich it. The arm 15 isswingably supported by ball bearings (not shown) provided at the upperand lower portions of a pivot portion 17. The ball bearings providedaround the pivot portion 17 are held by a carriage portion (not shown).

A magnetic head support mechanism is controlled by a positioning servodriving system. The positioning servo driving system comprises afeedback control circuit having a head position detection sensor (notshown), a power supply (not shown), and a controller (not shown). When asignal is supplied from the controller to the respective power suppliesbased on the detection result of the position of the magnetic head 13,the driving coil of the voice coil motor 19 and the piezoelectricelement (not shown) of the head portion are driven.

A cross sectional view of a conventional longitudinal recording diskmedium is depicted in FIG. 2. A longitudinal recording medium typicallycomprises a non-magnetic substrate 20 having sequentially deposited oneach side thereof an underlayer 21, 21′, such as chromium (Cr) orCr-alloy, a magnetic layer 22, 22′, typically comprising a cobalt(Co)-base alloy, and a protective overcoat 23, 23′, typically containingcarbon. Conventional practices also comprise bonding a lubricant topcoat(not shown) to the protective overcoat. Underlayer 21, 21′, magneticlayer 22, 22′, and protective overcoat 23, 23′, are typically depositedby sputtering techniques. The Co-base alloy magnetic layer deposited byconventional techniques normally comprises polycrystallites epitaxiallygrown on the polycrystal Cr or Cr-alloy underlayer.

The underlayer and magnetic layer are conventionally sequentiallysputter deposited on the substrate in an inert gas atmosphere, such asan atmosphere of pure argon. A conventional carbon overcoat is typicallydeposited in argon with nitrogen, hydrogen or ethylene. Conventionallubricant topcoats are typically about 20 Å thick.

It is recognized that the magnetic properties, such as Hr, Mr, S* andSMNR, which are critical to the performance of a magnetic alloy film,depend primarily upon the microstructure of the magnetic layer which, inturn, is influenced by one or more underlying layers on which it isdeposited. It is also recognized that an underlayer made of softmagnetic films is useful in perpendicular recording media because arelatively thick (compared to magnetic layer) soft underlayer provides areturn path for the read-write head and amplifies perpendicularcomponent of the write field in the recording layer. However, Barkhausennoise caused by domain wall motions in the soft underlayer can be asignificant noise source. Since the orientation of the domains can becontrolled by the magnetic anisotropy, introducing a magnetic anisotropyin the soft underlayer would be one way to suppress Barkhausen noise.When the magnetic anisotropy is sufficiently large, the domains wouldpreferably orient themselves along the anisotropy axis.

The magnetic anisotropy could be controlled in several ways in the softmagnetic thin film materials. The most frequently applied methods arepost-deposition annealing while applying a magnetic field and applying abias magnetic field during deposition. However, both methods can causecomplications in the disk manufacturing process.

A “soft magnetic” material is material that is easily magnetized anddemagnetized. As compared to a soft magnetic material, a “hard magnetic”material is one that neither magnetizes nor demagnetizes easily. Theproblem of making soft magnetic materials conventionally is that theyusually have many crystalline boundaries and crystal grains oriented inmany directions. In such metals, the magnetization process isaccompanied by much irreversible Block wall motion and by much rotationagainst anisotropy, which is usually irreversible. See Mc-Graw HillEncyclopedia of Science & Technology, Vol. 5, 366 (1982). Mc-Graw HillEncyclopedia of Science & Technology further states that the preferredsoft material would be a material fabricated by some inexpensivetechnique that results in all crystal grains being oriented in the sameor nearly the same direction. Id. Applicants, however, have found that“all grains” oriented in the same direction would be very difficult toproduce and would not be the “preferred soft material.” In fact,applicants have found that very high anisotropy is not desirable.

This invention describes how one can create magnetic anisotropy in softunderlayer by maximizing the effect of a magnetron field and minimizingmagnetostriction effect.

SUMMARY OF THE INVENTION

The invention provides a perpendicular magnetic recording medium havinghigh areal recording density exhibiting low noise. One way of achievingthis goal is to produce a magnetic film in the perpendicular magneticrecording medium with an easy axis substantially directed in atransverse direction to a traveling direction of a read-write head.

One embodiment is a recording device for perpendicular recordingcomprising a magnetic head and a recording medium, the recording mediumcomprising a substrate and a magnetic underlayer on the substrate, theunderlayer comprising an easy axis of magnetization directed in adirection substantially transverse to a traveling direction of themagnetic head. The underlayer could comprise a substantially radial ortransverse anisotropy.

The underlayer could comprise a soft magnetic material. The underlayerprovides a return path for a recording head. The underlayer could alsoamplify a perpendicular component of a write field in a recording layeroverlying the underlayer. Preferably, the underlayer would have lowmagnetostriction about 0, typically lower than 1-5×10⁻⁶. The underlayercould comprise a material selected from the group consisting of apermalloy, a CoZrNb alloy, a NiFe alloy and a FeAlN alloy. The recordingmedium could be a disk, a tape or any other device capable of recordingdata.

In one embodiment, the underlayer has the easy axis of magnetizationinduced by a magnetron field and the thickness of the underlayer couldbe about 200-400 nm.

Another embodiment is a method for manufacturing a magnetic recordingdisk for perpendicular recording, comprising applying a magnetron fieldand depositing an underlayer on a substrate, wherein the underlayercould comprise an easy axis of magnetization directed in a radialdirection of the magnetic recording disk. The method could furthercomprise heating the substrate. The step of depositing an underlayercould be by sputtering, wherein the sputtering could be a reactivesputtering. In one variation, the substrate is kept stationary duringthe depositing a magnetic underlayer, wherein a diameter of a magnetronsource producing the magnetron field is larger than a diameter of thesubstrate. In another variation, the substrate is rotated during thedepositing a magnetic underlayer, wherein a size of a magnetron sourceproducing the magnetron field is smaller or comparable to a diameter ofthe substrate and the substrate is placed off-center with respect to themagnetron source.

Another embodiment is a disk drive comprising a magnetic recording diskfor perpendicular recording, wherein the magnetic recording diskcomprises a substrate and a magnetic underlayer on the substrate,wherein the underlayer comprises an easy axis of magnetization directedin a radial direction of the magnetic recording disk.

Yet another embodiment is a magnetic recording disk for perpendicularrecording, comprising a substrate and means for providing a return pathfor a recording head. In this invention, means for providing a returnpath for a recording head comprises a soft magnetic layer having an easyaxis of magnetization directed in a radial direction of the magneticrecording disk or directed in a transverse direction of a travelingdirection of a magnetic head traveling over a tape or disk during areadwrite operation.

As will be realized, this invention is capable of other and differentembodiments, and its details are capable of modifications in variousobvious respects, all without departing from this invention.Accordingly, the drawings and description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a magnetic disk drive.

FIG. 2 is a schematic representation of the film structure in accordancewith a magnetic recording medium of the prior art.

FIG. 3 is perspective view of a magnetic head and a magnetic disk of aperpendicular recording disk medium.

FIGS. 4 a and 4 b are schematic diagrams of a magnetron source and disksubstrate in embodiments of this invention showing a fixed magnetronsource centered and off-centered from the center of a rotating disksubstrate.

FIG. 5 is a schematic representation of the film structure in accordancewith a magnetic recording medium of this invention.

FIG. 6 shows hysteresis loops of an as-deposited CoZr5Nb4 film measuredin radial and circumferential directions using a DMS vector VSM.

FIG. 7 shows the hysteresis loops measured in radial and circumferentialdirections of a FeAlN film.

DETAILED DESCRIPTION

A perpendicular recording disk medium, shown in FIG. 3, is similar tothe longitudinal recording medium depicted in FIG. 2, but with thefollowing differences. First, a perpendicular recording disk medium hassoft magnetic underlayer 31 of an alloy such as Permalloy instead of aCr-containing underlayer. Second, as shown in FIG. 3, magnetic layer 32of the perpendicular recording disk medium comprises domains oriented ina direction perpendicular to the plane of the substrate 30. Also, shownin FIG. 3 are the following: (a) read-write head 33 located on therecording medium, (b) traveling direction 34 of head 33 and (c)transverse direction 35 with respect to the traveling direction 34.

This invention provides magnetic recording media suitable for high arealrecording density exhibiting high SMNR. This invention achieves suchtechnological advantages by forming a substantially radial anisotropy ina soft underlayer. The underlayer is “soft” because it made of a softmagnetic material and it is called an “underlayer” because it residesunder a recording layer. A seedlayer, on the other hand, is a layerlying in between the substrate and the underlayer.

In a preferred embodiment, a magnetron field produces the radialanisotropy in the soft underlayer. In a magnetron, electrons generatedfrom a heated cathode move under the combined force of a radial electricfield and an axial magnetic field. By its structure, a magnetron causesmoving electrons to interact synchronously with traveling-wavecomponents of a microwave standing-wave pattern in such a manner thatelectron potential energy is converted to microwave energy with highefficiency.

The magnetron is a device of essentially cylindrical symmetry. On thecentral axis is a hollow cylindrical cathode. The outer surface of thecathode carries electron-emitting materials, primarily barium andstrontium oxides in a nickel matrix. Such a matrix is capable ofemitting electrons when current flows through the heater inside thecathode cylinder.

At a radius somewhat larger than the outer radius of the cathode is aconcentric cylindrical anode. The anode serves two functions: (1) tocollect electrons emitted by the cathode and (2) to store and guidemicrowave energy. The anode comprises a series of quarter-wavelengthcavity resonators symmetrically arranged around the cathode.

A radial dc electric field (perpendicular to the cathode) is appliedbetween cathode and anode. This electric field and the axial magneticfield (parallel and coaxial with the cathode) introduced by pole piecesat either end of the cathode, as described above, provide the requiredcrossed-field configuration.

The phrase “an easy axis of magnetization directed in a directionsubstantially transverse to a traveling direction of the magnetic head”means that the easy axis of magnetization is directed more toward adirection transverse to the traveling direction of the read-write headthan toward the traveling direction. Also, the phrase “a substantiallyradial or transverse anisotropy” means that the domains of the softmagnetic material of the underlayer are directed more toward a directiontransverse to the traveling direction of the read-write head than towardthe traveling direction.

Typically, when a magnetic recording medium is a tape, the tape travelsand the head is stationary. Therefore, “a traveling direction of themagnetic head” of a stationary head of a recording device in which themagnetic recording tape moves is the direction in which the head“travels” spatially with respect to the magnetic recording tape.

In one embodiment of this invention, fixed magnetron sources withinner-and-outer magnet poles are used in a single disk type sputteringmachine. Shunts are placed in the gap between the inner and outer magnetpoles. Soft magnetic films with low magnetostriction are sputtered usingthe above magnetron sources on supersmooth disk substrates. Lowmagnetostriction is obtained by choosing either proper material such aspermalloy and CoZrNb or proper sputtering conditions for the materialswith higher magnetostriction such as NiFe55 and FeAlN. The disksubstrates are kept either stationary (Example I) or rotating (ExampleII), depending upon the relative size of the magnetron source and thedisk substrate. In Example I, the diameter of the source is larger thanthat of disks, and disks are concentric with the source duringsputtering. In Example II, the size of magnetron source is smaller orcomparable to that of the disk, and vertical distance between the sourceand substrate is relatively small.

A disk substrate 41 is placed centered (FIG. 4 a) and off-centered (FIG.4 b) with a target 42, to minimize stress in the films. In FIG. 4 a, themagnet array in the circular magnetron is symmetrical and the disk andmagnetron are in concentric geometry. Therefore, no relative motionbetween magnetron and disk substrate is required. In another variationof the method shown in FIG. 4 a, the magnet array in circular magnetroncan be asymmetrical. Thus either the disk or magnetron should be rotatedto provide a radial field to the disk. However, as shown in FIG. 4 b,even if the disk is off-centered from the magnetron, a net field can beapplied in the radial direction of the disk when the disk is rotated.Offset should be adjusted to obtain a radial field on the whole disk,depending upon size of disk, magnetron pack and magnetron field profile.The soft-underlayers manufactured according to the current methods hadradial magnetic anisotropy induced by a field.

The term “induced” is used because magnetron field is external factorthat causes anisotropy. Unlike magnetic anisotropy caused bymagnetocrystalline or shape anisotropy, anisotropy formed by magnetronfield is considered as induced anisotropy.

Applicants recognized that the soft-underlayers manufactured accordingto the current methods have radial magnetic anisotropy induced by afield by considering all possible origins of magnetic anisotropy:

(1) Magnetocrystalline anisotropy: Sputtered thin films listed asexamples in this application form poly-crystalline microstructure.Crystallographic orientation of micro-crystals in the films is random inthe plane of disk. There is no reason why they form differently inradial and circumferential direction. Therefore, the disks of thisinvention do not have magnetocrystalline anisotropy.

(2) Shape anisotropy: This also does not explain the anisotropy betweenradial and circumferential direction of the disks of this invention.

(3) Stress anisotropy: Magnetostriction can cause stress-inducedanisotropy. However, the disks of this invention had very lowmagnetostriction effect and, therefore, there could be substantially nostress anisotropy in the disks of this invention.

(4) External field induced anisotropy: When an external magnetic fieldis applied during film deposition, it can create short-range orderingthat produces magnetic anisotropy in the direction of applied field.This method is used in manufacturing a read-write head of a recordingmedium. Similarly, this invention utilizes a magnetron field to apply afield in the radial direction of disks to produce radial anisotropy.

In accordance with embodiments of this invention, the substrates thatmay be used in the invention include glass, glass-ceramic, NiP/aluminum,metal alloys, plastic/polymer material, ceramic, glass-polymer,composite materials or other non-magnetic materials.

A preferred embodiment of a perpendicular recording medium of thisinvention is shown in FIG. 5. The thickness of soft magnetic underlayer52 is about 200-400 nm, and the thickness of magnetic layer 55 depositedon the underlayer is about 20 nm. In between the soft magneticunderlayer 52 and the magnetic layer 55 could be intermediate layers 53and 54 of thickness of about 5-10 nm. Protective layer 56 typicallycovers the magnetic layer 55.

A perpendicular recording medium of this invention comprises a softunderlayer and a recording layer. A soft underlayer should preferably bemade of soft magnetic materials and the recording layer shouldpreferably be made of hard magnetic materials. A soft underlayer isrelatively thick compared to other layers. Any layers between the softunderlayer and the recording layer is called interlayer or intermediatelayer. An interlayer can be made of more than one layer of non-magneticmaterials. The purpose of the interlayer is to prevent an interactionbetween the soft magnetic underlayer and recording layer. An interlayercould also promote the desired properties of the recording layer.Conventional (longitudinal) media do not have a soft magneticunderlayer. Therefore, the layers named as “underlayer,” “seed layer,”“sub-seed layer,” or “buffer layer” of longitudinal media are somewhatequivalent to the intermediate layer(s) of perpendicular media.

“Anisotropy” in the examples below was determined as follows. First, thehysteresis loops of the soft underlayer material were measured alongboth the radial and circumferential directions of the magnetic recordingdisk. For example, FIGS. 6 and 7 show representative hysteresis loops ofthe two soft underlayer materials of Examples 1 and 2, discussed below.From FIG. 6 one can see that external field of about 1 Oe can saturatethe underlayer used in the disk of example of FIG. 6 when a field isapplied in the radial direction, while about 20 Oe is necessary tosaturate the sample in circumferential direction. In other words, it iseasier to saturate the sample in radial direction than incircumferential direction. Thus, radial and circumferential directionsare called the easy and hard axis, respectively.

The underlayers of the disks whose hysteresis loops are shown in FIG. 6also have radial anisotropy. If there would be no anisotropy, thehysteresis loops for the radial and circumferential directions wouldsuperimpose on each other. The fact that these two hysteresis loops donot superimpose over one another indicates that the magnetic underlayermaterial has anisotropy. With reference to the hysteresis loops of FIG.6, “anisotropy” is determined as follows.

(1) A straight line that goes through the origin and represents theslope of the initial portion of the hard axis hysteresis loop passingthrough or near the origin is drawn.

(2) The saturated part of the easy axis hysteresis loop is extendeduntil it intersects with the line drawn in step (1).

(3) The field that corresponds to the intersection of the two linesdrawn in steps (1) and (2) is called Hk and it is a measure of theradial anisotropy of the soft magnetic underlayer sample.

In particular, the soft magnetic underlayers of the disks whosehysteresis loops are shown in FIGS. 6 and 7 have radial anisotropyvalues of 17 Oe and 30 Oe, respectively.

EXAMPLES

All samples described in this disclosure were fabricated with DCmagnetron sputtering except carbon films were made with AC magnetronsputtering.

Example I

A single-disk type of sputtering machine with multi-vacuum chambers wasused for fabricating CoZr5Nb4 alloy films as shown in FIG. 4 a. Thediameter of magnetron source and disk were 7 inch and 84 mm,respectively. The films were sputtered on either heated or unheatedsubstrates in argon gas pressure of 3 to 6 mtorr using 1 to 4 kW DCpower. The hysteresis loops of an as-deposited CoZr5Nb4 film measured inradial and circumferential direction using a DMS vector VSM are shown inFIG. 6. A “DMS vector VSM” is Model 10 VSM provided by DigitalMeasurement Systems. It is called vector VSM because the direction offield can be changed arbitrary with respect to sample.

The particular film of the disk whose hysteresis loop is shown in FIG. 6was sputtered on unheated disk substrate by applying 4 kW power at argonpressure of 3 mtorr. Hysteresis loops of the films sputtered indifferent conditions showed similar characteristics as shown in FIG. 6,due to low magnetostriction, which is insensitive to depositionconditions. The term “different conditions” means different sputteringconditions such as different power and pressure, i.e., a wide sputteringprocess window is available to obtain CoZrNb films with lowmagnetostriction and thus obtain radial anisotropy induced by magnetronfield. The term “similar characteristics” means that the soft underlayerhas very low coercivity (1 Oe or less) in both radial andcircumferential directions and it has a radial magnetic anisotropy oforder of 15 Oe. As-deposited CoZr5Nb4 films sputtered in any conditionshad radial anisotropy induced by a magnetron field.

This method could be used with NiFe alloys. Due to different dependencyof magnetostriction on the composition of the alloy, proper sputteringconditions that allow a magnetron field to take a full effect oninducing anisotropy are different for different compositions. Forexample, NiFe25 films that had almost zero magnetostriction showed thestrongest radial anisotropy when they were sputtered on an unheatedsubstrate with relatively low power, whereas NiFe40 films were radiallyanisotropic when they were sputtered on heated substrates.

Example II

A single-disk type of sputtering machine containing a multi-magnetronsource was used for fabricating FeAlN films as shown in FIG. 4 b. Thediameter of the magnetron source was 3 inch, and the diameter of diskwas 84 mm or larger. The films were sputtered on unheated substrates inthe mixture of argon and nitrogen gas by reactive sputtering using FeAlalloy target. Nitrogen gas flow % and total gas pressure was varied from0 to 15% and 3 to 15 mtorr, respectively. The DC power was fixed at 400W and offset was varied from about 0.25 to 1.55 inch. Here, offset isdefined as the horizontal distance between the center of the disk andthe center of the source. The disk substrates were rotated in 50 rpm.The orientation of magnetic anisotropy and the magnitude of theanisotropy field of the films showed very complicated dependency onnitrogen %, total gas pressure, and thickness. The films sputtered atlarge offset with low nitrogen flow % showed radial anisotropy inducedby magnetron field. FIG. 7 shows the hysteresis loops measured in radialand circumferential directions for a 400 nm thick film sputtered at 1.55inch offset. The film was sputtered at 2% nitrogen flow and total gaspressure of 3 mtorr, and showed radial anisotropy.

By this invention, radial magnetic anisotropy can be induced in softunderlayer films deposited on disk substrates by using a fixed poleDC-magnetron sputtering source. Magnetostriction effects on theanisotropy need to be minimized by choosing materials with lowmagnetostriction, or adjusting sputtering conditions.

The above description is presented to enable a person skilled in the artto make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

This application discloses several numerical range limitations. Personsskilled in the art would recognize that the numerical ranges disclosedinherently support any range within the disclosed numerical ranges eventhough a precise range limitation is not stated verbatim in thespecification because this invention can be practiced throughout thedisclosed numerical ranges. A holding to the contrary would “let formtriumph over substance” and allow the written description requirement toeviscerate claims that might be narrowed during prosecution simplybecause the applicants broadly disclose in this application but thenmight narrow their claims during prosecution. Finally, the entiredisclosure of the patents and publications referred in this applicationare hereby incorporated herein by reference.

1. A recording device for perpendicular recording comprising a magnetichead and a recording medium, the recording medium comprising asubstrate, a magnetic underlayer on the substrate and a perpendicularrecording layer, wherein the underlayer has an easy axis ofmagnetization induced by a magnetron field, and wherein the underlayercomprises the easy axis of magnetization directed in a radial directionof the recording medium.
 2. The recording device of claim 1, wherein theunderlayer comprises a soft magnetic material.
 3. The recording deviceof claim 1, wherein the underlayer provides a return path for arecording head.
 4. The recording device of claim 3, wherein theunderlayer amplifies a perpendicular component of a write field in theperpendicular recording layer overlying the underlayer.
 5. The recordingdevice of claim 1, wherein the underlayer has low magnetostriction. 6.The recording device of claim 1, wherein the underlayer comprises amaterial selected from the group consisting of a permalloy, a CoZrNballoy, a NiFe alloy and a FeAlN alloy.
 7. The recording device of claim1, wherein the recording medium is selected from the group consisting ofa disk and a tape.
 8. The recording device of claim 1, wherein athickness of the soft magnetic underlayer is about 200-400 nm.
 9. Amagnetic recording disk for perpendicular recording, comprising asubstrate, a perpendicular recording layer and means for providing areturn path for a recording head, wherein the means comprises anunderlayer that has the easy axis of magnetization induced by amagnetron field, and wherein the underlayer comprises an easy axis ofmagnetization directed in a radial direction of the magnetic recordingdisk.
 10. A magnetic recording disk for perpendicular recording, whereinthe magnetic recording disk comprises a substrate, a magnetic underlayeron the substrate and a perpendicular recording layer, wherein theunderlayer comprises an easy axis of magnetization directed in a radialdirection of the magnetic recording disk, and wherein the underlayer hasthe easy axis of magnetization induced by a magnetron field.